There is conventionally known a press machine in which an upper end of a connecting rod is connected to an eccentric portion of a main shaft and a slide is attached to a lower end of the connecting rod without a plunger interposed therebetween (see, for instance, Patent Literature 1). Since no plunger exists between the connecting rod and the slide, a structure of the press machine can be simplified and the total height of the press machine can be lowered.
These days, an electric servo motor is frequently used as a driving source for a main shaft. When a press machine uses a servo motor, a slide motion is advantageously controllable as desired by adjusting, for instance, the drive speed and/or the driving start position of the servo motor. For instance, in a typical press machine, a waiting position of a slide usually corresponds to a top dead center. However, when a servo motor is used, the waiting position of the slide may be set at a position with the main shaft being normally rotated by a predetermined crank angle θ.
In such a case, for instance, a reverse motion and a reciprocating (oscillatory) motion are possible. In the reverse motion, the slide is first moved to a bottom dead center from the waiting position by normally rotating the main shaft and then returned to the original position (i.e., the waiting position) from the bottom dead center by reversely rotating the main shaft. In the reciprocating motion, after moved to the bottom dead center, the slide is continuously moved to another waiting position that is away from the top dead center by an amount corresponding to an angle minus θ by normally rotating the main shaft so that the slide is moved back to the original waiting position corresponding to the angle θ from the waiting position corresponding to the angle minus θ via the bottom dead center to press the next workpiece.